It was a death that woke a nation. On Sept. 18, Pavlos Fyssas, a popular left-wing musician, was cornered in an Athens suburb by a gang of thugs and fatally stabbed. Once it emerged that the alleged killer was an active member of Golden Dawn, a far-right nationalist group that has recently become a major player in Greek politics, the backlash was swift.

Leading Golden Dawn members, including some parliamentarians, were arrested and charged with running a criminal organization. Prime Minister Antonis Samaras vowed to "eradicate" any trace of fascism in Greek...